Magnets formed from Nd--Fe--B powders find applications in a wide spectrum of industries, including computer hardware, automobiles, consumer electronics and household appliances. The magnets from these powders often suffer from expansion defects due to the presence of Nd.sub.2 O particles in the powders. The Nd.sub.2 O.sub.3 particles are impurities in the magnet--they may come from the powders that form the magnet or they may be formed in the magnet during the process by which the magnet is made from the powders. The Nd.sub.2 O.sub.3 particles in a magnet adversely affect the properties of the magnet because these particles react with moisture to form Nd(OH).sub.3 : EQU Nd.sub.2 O.sub.3 +3H.sub.2 O.fwdarw.2Nd(OH).sub.3.
The density .rho. of Nd(OH).sub.3 is less than the density of Nd.sub.2 O.sub.3 : EQU .rho.(Nd.sub.2 O.sub.3)=7.28 g/cm.sup.3, EQU .rho.(Nd(OH).sub.3)=5.60 g/cm.sup.3.
As a result of this decrease in density after the Nd.sub.2 O.sub.3 particles react with moisture, there is an expansion that is sufficient to cause an eruption in the magnet. If the air gap is very small, the resulting eruption may stall the motor as a result of the defect.